1. Field of the Invention
The present invention relates to a method and apparatus for detecting a spread code from a perch channel of the wide band CDMA (Code Division Multiple Access) by a perch channel detecting circuit, and, in particular, to a method and apparatus for detecting a spread code from a perch channel which simplifies the timing control between base stations.
This application is based on Japanese Patent Application No. Hei 11-259705, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In a perch channel detecting circuit, a conventional method and circuit for detecting a spread code perform timing control between base stations so that the spread timings for transmitting the perch channels do not overlap. Accordingly, the perch channel assigned to the base station can be uniquely determined, based on the spread timing detected using a known code.
A CDMA transmitting/receiving system will be explained with reference to FIG. 5.
A transmitter which is a base station spreads a logic symbol P with a spread code A using a spreader 21, spreads a logic symbol Q with a spread code B using a spreader 22, and transmits them with carrier waves at the same frequency from an antenna. The transfer rate of the spread codes is several ten to hundred times of the transfer rate of the logic symbols. The cycle of the spread code is called a chip.
A receiver which is a mobile station despreads (inverse spreading) the received carrier waves with the spread code B using a despreader (inverse spreader) 23 to extract the logic symbol Q, and despreads the carrier waves with the spread code A using a despreader 24 to extract the logic symbol P.
The CDMA transmitting/receiving system cannot accurately carry out the dispreading when the receiver does not accurately know the spread timing used by the receiver which is the base station. For example, when the spread timings are different by one chip, the receiver cannot receive the signal transmitted from the base station.
Because there are a plurality of spread codes for respective base stations, the receiver cannot know the spread code used by the base station to which the receiver is to connect. When the receiver does not specify the spread code used by the base station, the receiver cannot receive any information from the base station. That is, the connection with the base station cannot be established, and communication is impossible.
A perch channel function solves this problem. The perch channel function allows the receiver to receive the base station information such as the spread code, with which the base station spreads the logic symbol, and the spread timing. According to the perch channel function, the receiver retrieves the perch channels, and can obtain various information of the base station, such as the spread code, and the spread timing. Thus, the receiver can achieve radio communication with the base station.
Since there are a number of spread codes, and the codes are long, it takes much time to detect the specified candidate spread code by despreading the signal with all the spread codes. To solve this problem, a number of the candidate spread codes are divided into a plurality of groups, which are called spread code groups, and one of the spread code groups is specified, thereby shortening the time required to detect the spread code.
In first step of the perch channel function, the spread timing is obtained, based on the known code. In second step, the spread code group in connection with the spread timing is obtained. In final third step, one of the spread codes in the obtained spread code group is obtained.
Referring to FIG. 6, a special symbol, which is called a long code mask symbol, is spread, as the unique short code corresponding to the spread code group, in each time slot which is included in each radio frame in the perch channel. The information of the spread code group is obtained by detecting the spread short code of the symbol.
Referring to FIG. 7, a conventional perch channel detecting circuit 110 comprises a spread timing detector 11, a spread code group correlative electric power calculator 12, a spread code group maximum correlative electric power detecting determining device 113, a spread code/frame timing detector 15, a report information despreader 16, a RAM I/F (RAM interface) 117, a controller 118 for controlling these devices, and a RAM (Random Access Memory) 119 which is an external storage device.
Referring to the flowchart of FIG. 8, the spread timing detector 11 of the perch channel detecting circuit 110 receives the perch channel (step S1), and detects the spread timing with the known code by control of the controller 118 (step S2).
Subsequently, the spread code group correlative electric power calculator 12 calculates the correlative electric powers of a plurality of the candidate spread code groups by control of the controller 118. From the calculation results, the spread code group maximum correlative electric power detecting determining device 113 determines the candidate spread code group having the maximum correlative electric power as the detection object, and records it in the RAM 119 (step S101).
Subsequently, the spread code/frame timing detector 15 detects one of the spread codes which are included in the recorded candidate spread code group having the maximum correlative electric power by control of the controller 118 (step S102).
In another example of a process for determining a spread code group in FIG. 8, the correlative electric power of a first code group is calculated (step S111), and is recorded as the maximum value (step S112). Then, the correlative electric power of the next code group is calculated (step S113), and is compared with the maximum value (step S114). Then, when the determination in step S115 is YES, the maximum value is updated to the greater value (step S116). These steps are repeated until the process for the last spread code group is completed (until in step S117 the determination is YES).
The above-mentioned conventional perch channel circuit and method requires a particular timing control between the base stations, that is, must avoid the overlap of the spread timings for transmitting the perch channels assigned to the respective base stations.
The reason for this is that the correlative electric powers of the spread code groups are calculated, and that the spread code group having the maximum correlative electric power value is selected, to thereby uniquely determine the perch channel, which is assigned to the base station, with the spread timing which is firstly detected by the known code according to the perch channel function.
It is therefore an object of the present invention to provide perch channel detecting circuit and method which can detect the spread codes even while the detected spread timings of the base stations overlap, eliminating the particular timing control between the base stations.
The method for detecting a spread code from a perch channel of the wide band CDMA (Code Division Multiple Access) by a perch channel detecting circuit, according to the present invention, comprises the steps of: detecting a spread timing from the perch channel; calculating correlative electric power values of a plurality of candidate spread code groups, based on the detected spread timing; comparing the correlative electric power values with a predetermined threshold value; selecting the spread code groups whose correlative electric power values are above the threshold value; and retrieving a spread code in the selected spread code group. The method may further comprises the steps of: extracting the calculated correlative electric power value of the candidate spread code group one by one; determining, one by one, whether the correlative electric power value of the candidate spread code group is above the threshold value; selecting the candidate spread code group when its correlative electric value is above the threshold value; aborting the candidate spread code group when its correlative electric value is not above the threshold value; repeating the determinations until the last candidate spread code groups; and proceeding to step of retrieving a spread code in the selected spread code group.
According to the method of the present invention, even when the same spread timing are used by neighboring base stations, the candidate spread code groups whose correlative electric power values are comparatively high but are not the maximum can be selected.
Specifically, the apparatus for detecting a spread code from a perch channel of wide band CDMA, according to the present invention, comprises: a spread timing detector for detecting a spread timing from the perch channel; a spread code group correlative electric power calculator for calculating correlative electric power values of a plurality of candidate spread code groups, based on the detected spread timing; a threshold value holder for holding a predetermined threshold value; a determining and recording device for determining and recording the spread code groups whose correlative electric power values are above the threshold value; and a spread code detector for detecting a spread code from the selected spread code groups.